The present invention relates to a method of forming and joining conductive oxide layers and, more particularly, the method of forming and the resulting juncture of the anode, cathode, electrolyte and support members in a tubular, planar or monolithic type solid oxide fuel cell.
Fuel cells of the type pertaining to the present invention, are exemplified by the disclosures of U.S. Pat. No. 4,598,028, "High Strength Porous Support Tubes for High Temperature Solid Electrolyte Electrochemical Cells", and U.S. Pat. No. 4,799,936, "Process of Forming Conductive Oxide Layers in Solid Oxide Fuel Cells". In these and similar fuel cells, fuel is absorbed at an anode, oxidant is absorbed at a cathode, and the fuel and oxidant react in the electrolyte.
In solid oxide fuel cells (SOFC), and in particular cells of the monolithic design (MSOFC) such as described in U.S. Pat. No. 4,799,936, the electrolyte is composed of a thin layer of yttria stabilized zirconia; the anode or fuel electrode is a cermet of nickel and zirconia; the cathode or air electrode is a strontium doped lanthanum manganite; and the interconnector, or support structure, is magnesium (or strontium) doped lanthanum chromite.
In the presently favored MSOFC, the generator is made up of a series of "stacks", each stack being made of a corrugated triplex layer of fuel electrode-electrolyte-air electrode, and a planar support layer of anode-interconnected-cathode, or, the corrugated layer may be sandwiched between two interconnector support layers or sheets. Fuel and oxidants must be physically separated by the impervious interconnector and electrolyte. These materials are approximately 95 percent of theoretical density. The fuel and air electrode must be porous enough to allow, by diffusion, both fuel and air to the electrodes and are approximately 70 percent of theoretical density.
In fabricating the MSOFC, the entire modular structure of corrugated layers, planar interconnector and the modular housing and manifold are initially assembled in the "green" or unsintered state. The entire modular structure is then debound at a relatively low temperature, so that decomposition and removal of organic constituents as a gas phase is achieved. The debound modular structure is then sintered as a unit. The material integrity of the monolithic envelope and the intrinsic electronic structure of the triplex electrolyte and interconnector members are of major significance in determining the working efficiency of the module.
Internal cracking, triplex layer lift off, delamination or bubbles have been the cause of a large number of failures in the initial fabrication trials on the MSOFC stack. Autopsies of failed stacks have shown that the possible causes of failure arise from debinding or sintered mismatch upon heating, and differential thermal expansion mismatch on temperature cycling. Thus, one of the greatest inherent problems associated with the ceramic process of debinding, is the achievement of temperature homogeneity throughout the ceramic system. Any temperature differential within the same material or different rates of decomposition between dissimilar materials will cause stress related areas within the structure.
As a result of these types of stresses, separation of contact points and macro separation of interfacing surfaces at later stages in the fabrication process or during in-cell operation are likely to occur. This degraded condition adversely affects performance and is a major obstacle in the further development of SOFC, and particularly, MSOFC.